In work vehicles such as bulldozers the output of the engine is transmitted to the driving wheels (sprockets) via a drive train. A torque converter, a lockup clutch which locks up the torque converter, and a transmission are provided in the drive train of the engine. A forward clutch corresponding to a forward running stage, a reverse clutch corresponding to a reverse running stage, and speed stage clutches corresponding to each speed stage are provided in the transmission. A running operation device is provided at the operator's seat on the working vehicle. The running operation device comprises a forward/reverse operation lever and a shift switch. A forward operation command signal and a reverse operation command signal are output in accordance with the operation position of the forward/reverse operation lever. A speed stage shifting command signal is output in accordance with the operation of the shift switch.
A forward clutch is engaged (connected) when it is selected in response to the forward operation command signal. A reverse clutch is engaged (connected) when it is selected in response to the reverse operation command signal. One of the speed stage clutches is selected in accordance with the speed stage shifting command and the selected speed stage clutch is engaged. Thus the engine power is transmitted to the driving wheels via the torque converter or the lockup clutch, the selected forward clutch or reverse clutch, and the selected speed stage clutch.
This type of mechanism having speed stages cannot avoid shift shock.
Shift shock derives from torque variations generated when the input and output sides of the clutch engage (are connected) and are due to matching, by sliding friction members across each other, the difference arising in the selected clutch (transmission element) between the revolution speed on the clutch input side and the clutch output side.
Various approaches are provided in conventional technology for suppressing shift shock as shown in for example patent documents 1 and 2.
Patent document 1: Japanese Patent Application Laid-Open No. 7-174220
Patent document 2: Japanese Patent Application Laid-Open No. 2000-97323
Patent document 1 discloses a control method for a hydraulic shift device for a running vehicle such as a tractor. For example, shifting is performed as shown in FIG. 10(A) when in forward running.
More specifically, first the engagement pressure of the forward clutch, which corresponds to the forward running stage is lowered to just above zero pressure, as shown by the solid line of signal A in FIG. 10(A), when a speed stage shifting command is output in response to the operation of running operation device. Then the pre-shift speed stage clutch corresponding to the speed stage before the shift is disengaged (released), as shown by the alternate long and short dash line of signal B in FIG. 10(A), during the period when the pressure of the forward clutch is lowered to just above zero. Meanwhile the post-shift speed stage clutch, corresponding to the selected speed stage after the shift, is connected (engaged) as shown by the alternate long and two short dashes line of signal C in FIG. 10(A). In this manner the engagement pressure of the running stage clutch (forward clutch) is lowered at the time of shifting in patent document 1, hence the shift shock at the time of shifting the speed stage clutch is absorbed by the running stage clutch (forward clutch).
On the other hand the invention described in patent document 2 lowers shift shock by having a measured speed of the work vehicle at the time of shifting approach a theoretical running speed. Up-shift when in forward running is performed for example as shown in FIGS. 10(B) and 10(C).
More specifically, when a speed stage shift command (up-shift command) is output from the running operation device at time t1, as shown in FIG. 10(C), at that time t1 the forward clutch is completely disengaged (released), this disengaged state continuing until time t2. The speed stage is shifted from the pre-shift speed stage (a low-speed speed stage) to a post-shift selected speed stage (a high-speed speed stage) during the period between time t1 in which the forward clutch is completely disengaged and time t2. The forward clutch is feedback controlled at time t3 by having the measured speed of the vehicle (the solid line in FIG. 10(B)) approach the theoretical running speed (the dashed line in FIG. 10(B)) calculated from the engine revolution speed and a target shift position. For example when the measured speed of the vehicle trends towards deviating from the theoretical running speed, a fixed jerk value is provided causing the measured value of the vehicle to trend towards the theoretical running speed. Note that the jerk value is proportional to the differential value of the clutch operation pressure and is an index value for evaluating shift shock. Additionally, by performing control to maintain acceleration at a fixed value the measured speed of the vehicle is matched to the theoretical running speed. Such control of clutch pressure of the forward clutch is performed until time t5 in which the measured speed of the vehicle and the theoretical running speed match. At time t5 the forward clutch is completely connected (engaged).